The Fire suffered an ugly loss in Philadelphia on Saturday and headed to San Jose without Bastian Schweinsteiger, Juninho and David Accam.
On top of that, the day began with the two teams directly ahead of the Fire in the Eastern Conference standings, New York City FC and Atlanta, both picking up wins.
However, losses by Montreal and New England earlier on Wednesday meant the Fire could clinch a Playoff Berth with a win or draw at San Jose.
Philadelphia used fullbacks to great effect as well on Saturday, and the Union's pair of Fabinho and Keegan Rosenberry ultimately outshined Vincent and Polster.
Rosenberry's assist came after a corner kick was only partially cleared, but he did make a nice move to give himself space for the cross and put in a good ball.
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